The number of echoes combined into a single image or image set in multiple echo imaging sequences or echo-train techniques such as rapid acquisition with relaxation enhancement (RARE), fast-spin echo (FSE), and turbo spin-echo (TSE). In RARE imaging, the ETL typically equals the acceleration factor.
As for tooth pain, it is caused by a reaction of the nerves inside a tooth's pulp chamber with the severity dependent upon the type and degree of the stimulus. What follows are some examples of symptoms you may be feeling and their possible causes.
Pain: Lasting sensitivity and more constant awareness of hot or cold foods without recent dental work. Meaning: The pulp is likely infected, the tooth may be cracked or chipped, or the beginning of gum disease. Pain: Temporary sensitivity to hot or cold foods after recent dental treatment. Meaning: Dental work can cause pulp inflammation which would be an intense but very short pain.